1. Field of the Invention
The present invention relates to a V-ribbed belt for power transmission wound to, for example, a DL (Damper & Limiter) pulley used for air conditioner (A/C) compressors of automobiles, and belongs to a technical field for preventing belt breakage caused due to locking of the pulley.
2. Background Art
Conventionally, auxiliaries and the like of an automobile are driven in such a manner that power from an engine crank shaft is transmitted through a transmission belt, such as a V-ribbed belt or the like. The V-ribbed belt is in contact on the side faces of rubber ribs thereof with a V groove face of the pulley to generate frictional force utilized for power transmission. In general, the winding angle to a pulley and the tension of a belt are determined from the engine layout, and the apparent coefficient of dynamic friction of the belt to the pulley is determined accordingly. When the apparent coefficient of dynamic friction is large, noise is generated. To tackling this problem, as indicated in Japanese Patent Publication No. 6-21607, a V-ribbed belt is well known in which short fibers are blended in the rubber ribs and part thereof are allowed to be protruded from the side faces of the rubber ribs (hereinafter, these short fibers are called protruded short fibers) to reduce the contact area between the side faces of the rubber ribs and the V groove face of the pulley, thereby reducing the frictional force.
Recent demands for lightweight and compact automobile engines leads to major employment of a generally-called serpentine belt drive in which engine auxiliaries are driven through a single belt. In association therewith, it grows more significant than ever before to provide countermeasures to prevent belt breakage. Heretofore, two or three belts were wound to one crank pulley to drive different auxiliary groups, and therefore, not all of auxiliary groups were stopped upon breakage of one of the belts. In contrast, belt breakage results in stop of every auxiliary group in the serpentine belt drive. This may leads to significant trouble in running, for example, trouble in engine seizing.
In view of fuel saving, power consumption saving, accelerating ability, shock reduction, and the like, recently, variable capacity compressors requiring no on-off operation mechanism of a clutch mechanism have been employed as air conditioner (A/C) compressors of automobiles. A torque limiter mechanism for preventing breakage of a V-ribbed belt is incorporated in the variable capacity compressors. An operation process of the torque limiter mechanism is as follows. Though some factor locks (fixes non-rotatably) the drive shaft of the compressor, the belt is forcedly driven still by rotation of the crank pulley (a drive pulley) mounted on the engine crank shaft, thereby causing belt to slip on a driven pulley, for example, a DL pulley. Strictly, the side faces of the rubber ribs slips on the V groove face of a pulley (hereinafter, slip start is called belt slip and before and after it are called before belt slip and after belt slip, respectively), so that a torque in the peripheral direction applied to the DL pulley increases. When the torque exceeds a predetermined value of the torque limiter mechanism, the torque limiter mechanism operates. The operation of the torque limiter mechanism breaks a limiter portion (breaking portion) thereof connecting the DL pulley and the drive shaft of the compressor to allow the DL pulley to idle, so that the belt is inhibited from slipping on the DL pulley. This mechanism prevents belt breakage caused due to heat generated at the slip face of the side faces of the rubber ribs against the pulley V groove face of the pulley.
In conventional belts, the apparent coefficient of dynamic friction of the side faces of the rubber ribs against the V groove face of the pulley is set low for suppressing noise generation by increasing the number of the protruded short fibers. Accordingly, it takes time until the influence of the short fibers is ceased though the number of the protruded short fibers is reduced after belt slip due to abrasion, fall-off, and breakage. Namely, a time period is long until the apparent coefficient of dynamic friction is increased by an increase in contact area between the side faces of the rubber ribs and the V groove face of the pulley, and therefore, belt breakage is caused by heat generated at the slip face before the apparent coefficient of dynamic friction reaches a value at which a torque necessary for limiter operation is generated.